Printed circuit boards (PCBs) generally comprise a composite of organic and inorganic materials with external and internal metal traces, permitting assembled electronic components to be mechanically supported and electrically connected. Electronic components are assembled on at least one of the outer metal layers of the typical printed circuit board. On a single-sided printed circuit board, for example, electronic components are assembled on only one of the outer metal layers, referred to as the primary side of the printed circuit board. The opposite side of such a printed circuit board is called the secondary side.
After the electrical components have been positioned on the primary side via, for example, a pick-and-place machine (not shown), the single-sided printed circuit board typically undergoes a single soldering stage. Alternatively, electronic components can be assembled on both the primary side and the secondary side of a double-sided printed circuit board. To assemble electrical components on both sides, double-sided printed circuit boards must typically undergo multiple soldering stages.
Today, a significant number of electrical components are surface mounted to printed circuit boards. Surface-mounted components are positioned on metal pads or lands, which are formed on the outer metal layers of the printed circuit board, by applying solder paste to the relevant lands before positioning the surface-mounted components. The solder paste liquefies during a manufacturing stage called solder reflow (or more simply reflow), and the surface tension of the liquefied solder enables the surface-mounted components to align with, and to couple with, the relevant lands on the printed circuit board. Thus, when the solder cools and solidifies, the surface-mounted components each are assembled on the printed circuit board.
To more efficiently utilize printed circuit board real estate, double-sided printed circuit boards are used with surface-mounted components assembled on both the primary side and the secondary side of the printed circuit board. The surface-mounted components first are assembled, i.e. positioned and reflowed, on an upward-facing side, such as the primary side, of the printed circuit board. The double-sided printed circuit board then is flipped over such that the opposite side, such as the secondary side, faces upwardly, and additional surface-mounted components are positioned on the secondary side. After the additional surface-mounted components have been positioned, the double-sided printed circuit board undergoes a second reflow stage to liquefy the solder on the upward-facing secondary side. During the second reflow stage, however, the entire double-sided printed circuit board is heated, and the solder on the primary side of the double-sided printed circuit board also liquefies with the surface-mounted components assembled on the primary side facing downwardly in opposition of gravity.
Unlike through-hole components, most surface-mounted components do not include any retaining devices, such as through-hole conductive pins and/or locating posts, to maintain component placement during the second reflow stage. Although adequate to maintain component placement on the upward-facing secondary side of the double-sided printed circuit board, the surface tension of the liquefied solder during the second reflow stage generally is insufficient to maintain component placement for heavier surface-mounted components previously assembled on the downward-facing primary side. Due to the effects of gravity, one or more of the surface-mounted components previously assembled on the downward-facing primary side can separate from or become misaligned with the relevant lands when the solder again liquefies during the second reflow stage.
To retain these larger surface-mounted components, adhesives presently are applied to the double-sided printed circuit board before the surface-mounted components are positioned. Automatic adhesive application, however, requires additional equipment, which must be purchased and maintained; whereas, manual application of the adhesive results in unpredictable quality and increased labor expenses.